In the treatment of vascular disease such as arteriosclerosis, intracoronary stent placement is a common adjunct to balloon angioplasty. Stents can eliminate vasospasm, attach dissections to a vessel wall, reduce negative remodeling, and maintain vessel patency. Stents, however, can also cause undesirable side effects. For example, the continued exposure of a stent to blood can lead to thrombus formation and the presence of a stent in a blood vessel can over time cause the blood vessel wall to weaken creating the potential for arterial rupture or formation of aneurisms. A stent can also become overgrown by tissue after its implantation such that its utility is diminished or eliminated while its continued presence may lead to a variety of complications such as the foregoing.
To ameliorate the above situation, stents can be fabricated from materials that are biodegradable and, if desired, bio-absorbable. The material selected must be not only biocompatible and biodegradable, it must have sufficient mechanical properties required of a stent. Such mechanical properties include, among others, sufficient strength to withstand the stresses to which the device will be subjected such as radial flexibility should the device be expandable as in the case of a balloon expandable stent and longitudinal flexibility to allow it to be advanced through a contorted vasculature and to adapt to a non-linear deployment site.
The above-described properties have been achieved at least in part using polymers such as polylactic acid, poly(lactic acid-co-glycolic acid) and polycaprolactone. These materials, however, typically biodegrade by bulk erosion which can result in large particles breaking away from the degrading stent. These particles, when released into the bloodstream, may cause emboli and/or other complications.
What is needed are biocompatible, biodegradable polymers that have the mechanical properties required of a stent and that biodegrade by surface rather than bulk erosion. Such polymers should also be useful to coat implantable medical devices for use as drug delivery systems since the lack of a bulk erosion propensity should drastically reduce if not eliminate flaking off of large particles of the coating when implanted in a patient. The current invention provides compositions comprising such polymers.